In making the announcement, scientists were quick to add that the Moon has not shrunk by much, that the shrinking may have occurred over a billion years, and that the Moon will not shrink out of view in the future.

“The kind of radius change and shrinking we’re describing here is so small that you would never notice it,” said Thomas R. Watters of the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum, during a NASA-sponsored telephone news conference on Thursday.

Dr. Watters and his colleagues deduced the Moon’s diminishing size from cracks on the surface seen in images taken by the National Aeronautics and Space Administration’s Lunar Reconnaissance Orbiter.

As the Moon’s core has cooled and contracted, the outer crust fractured into faults, forming ridges as one side of the fracture slid on top of the other. The same cooling and shrinking occurs within all planetary bodies; NASA’s Messenger spacecraft recently observed similar — and much larger — fractures on the planet Mercury.

The largest of the Moon ridges is about 300 feet high and stretches for several miles, and some had been seen during the Apollo Moon missions. During Apollo 17, Eugene Cernan and Harrison Schmidt even drove to one in their lunar rover.

But most of the ridges are much smaller and shorter, only tens of yards high, which is why they had escaped notice until the Lunar Reconnaissance Orbiter and its sharp-eyed camera arrived in orbit last year.

Photo

As a result of the moon’s shrinkage, its brittle crust ruptured and thrust faults pushed crustal materials up the side of a crater (at arrows) and formed distinctive landforms.Credit
NASA/GSFC, via Arizona State University, via Smithsonian

Over the eons, the shrinkage for the Moon was only about 200 yards, the length of two football fields, out of a diameter of 2,160 miles.

The scientists believe that the ridges are young — for planetary geologists, a billion years old is young compared with the 4.5-billion-year age of the solar system — because the ridges cut across small, young craters, but no large craters appear on top of any of the ridges. Large impacts are much rarer and likely to be much more ancient. The ridges also look freshly carved in the moonscape.

Some ridges could be much younger than a billion years, Dr. Watters said, and the shrinking and cracking could be still occurring today.

Michael Wargo, the chief lunar scientist at NASA headquarters, said the findings, which appear in Friday’s issue of the journal Science, were more evidence that the Moon was much livelier than its reputation as a cold, unchanging world. “We’re now talking about the Moon in a completely different way,” Dr. Wargo said.

A curious lack of older, larger shrinkage cracks may put constraints on current ideas for the origin of the Moon, that it coalesced out of molten wreckage left behind when a Mars-size object slammed into the Earth.

Dr. Watters said the observations did not argue against the impact theory, but “our results are really much more consistent with a cooler initial starting temperature for the Moon, a starting temperature that did not allow the entire Moon to melt.”

A version of this article appears in print on August 20, 2010, on Page A15 of the New York edition with the headline: Over a Billion Years, Scientists Find, the Moon Went Through a Shrinking Phase. Order Reprints|Today's Paper|Subscribe